In the single face corrugation machine, the corrugations are formed between two heated serrated rollers which rotate in opposite directions and which are pressed against one another along the pitch line. These serrated rollers are cylindrical rollers with serrations which are formed in the roller shell, which correspond to the desired shape of the corrugation of the paper to be corrugated, which are parallel to the axes of said rollers and which are of equal depth on both said rollers. The serration tips, i.e. the tips of the said serration-like teeth of each roller, press the paper against the serration trough, i.e. the serration roots of the respective mating roller, thereby imparting a permanent corrugation to it. The profile form of the serrated rollers corresponds to their task and the teeth of a serrated roller are thus fundamentally different from gear teeth.
In the case of gears, whose task is to transmit or convert a speed and torque, this transmission is accomplished in principle by the gears engaging on the contacting gear flanks. A considerable amount of clearance is provided in principle between the tooth tip and tooth root in order to prevent the gears from seizing. Circumferential forces are transmitted between adjacent tooth flanks by positive engagement.
In the case of serration-type teeth, however, whose task is to corrugate a web of paper or other material passing therethrough, there must not be any flank/paper web/flank contact. In order to obtain a stable paper corrugation, the serration-type teeth are pressed against the serration troughs under high linear pressures exerted on the interposed paper and, in addition, a considerable temperature is also used. The one serrated roller, which is not driven by an external means, is carried along, i.e. it is rotated by the serrated roller which is provided with a drive means. This occurs substantially by positive engagement across the paper which is pinched between the tooth tip and tooth root. A substantial torque transmission would not be possible with serration-type teeth and is not even required, since the only counter-torque which exists is the torque caused by the friction of the serrated roller mounting.
A comparison with normal gears shows that the laws for gear toothing which were developed for gears cannot be applied to serration-type teeth. Down to the present day, serration-type teeth are composed of circles and straight lines in accordance with the desired shape of the corrugated paper. The periodic variations in axial spacing between the centers of the two serrated rollers which occur in the rhythm of twice the tooth frequency are put up with because the rollers do not roll off each other perfectly and the variations are in the magnitude of a few hundredths of a millimeter. The serrated roller, which is lever-mounted and pressed against the other by hydraulic cylinders, yields by this amount and thus reduces and increases periodically the amount of sag accordingly.
The conditions in the glueing zone between the one serrated roller and the pressure roller are not any different and may even be more unfavourable. Since the latter has a circular shape, every serration will exert a jolt onto the pressure roller each time it passes over the two paper webs which are interposed between the two rollers in this zone. This will cause a change in the spacing from the center of the adjacent serrated roller in the rhythm of the tooth frequency. The magnitude of the variation in axial spacing corresponds to the arc height between the chord disposed above two adjacent teeth and the external diameter of the serrated roller. It is approximately twice as large as that between the two afore-mentioned serrated rollers and can approach almost 1/10 of a millimeter in the case of coarse toothing.
Even if the magnitude of the change in axial spacing between the pressure roller and serrated roller and between two serrated rollers is small, the dynamic forces which occur are very considerably due to the high frequency which can approach a magnitude of 800 Hz and due to the large masses of the rollers (about 1000 kg per roller). High pressures must be used for this reason alone in order to maintain permanent contact on both sides on the paper passing therethrough and in order to ensure clean impressions and glueing. When the pressures can no longer compensate sufficiently for the dynamic jolts which increase greatly at higher machine speeds, the result is a temporary raising of the rollers, poor impressions and glueing, as well as the feared "high-low" phenomenon in which only every other corrugation has a good impression and is correctly glued. Moreover, the periodical variations in axial spacing are a principal cause of the loud machine noise in the case of single face corrugating machines.